The effectual fervent prayer of a righteous man availeth much. (James 5.16b)
It is reported that Mary, Queen of Scots, confessed that she feared the prayers of John Knox more than an army of ten thousand men.
Puritan prayer warrior and catechist Thomas Lye (d. 1684) said in his second ejection sermon preached on August 17, 1662 that "I had rather stand against the canons of the wicked, then against the prayers of the righteous."
John Whitecross, The Shorter Catechism Illustrated From Christian History and Biography, p. 157:
Theodorus, speaking of Luther, says, 'Once I overheard him in prayer; but oh! with what life and spirit did he pray! It was with so much reverence as if he were speaking to God; yet with so much confidence as if he had been speaking to a friend.' Perceiving the interest of religion to be low, he be took himself to prayer. Rising off his knees, he came out of his closet triumphantly saying to his friends, 'We have overcome, we have overcome.' At which time it was observed, there came out a proclamation from Charles V that none should be further molested for their profession of the gospel.
Martin Luther, Table Talk CCCXXXIII:
It is impossible that God should not hear the prayers which with faith are made in Christ, though he give not according to the measure, manner, and time we dictate, for He will not be tied. In such sort dealt God with the mother of Saint Augustine; she prayed to God that her son might be converted, but as yet it would not be; then she ran to the learned, entreating them to persuade and advise him thereupon. She propounded unto him a marriage with a Christian virgin, so that thereby he might be drawn and brought to the Christian faith. Nothing worked. But when our Lord God came thereto, He came to purpose and made of him such an Augustine, that he became a great light to the church. Saint James says: "Pray for one another, for the prayer of the righteous availeth much." Prayer is a powerful thing, for God has bound and tied Himself thereunto.